FXUS66 KSGX 212009 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 109 PM PDT Thu Oct 21 2021 .SYNOPSIS... Warmer conditions are in store today, with weak high pressure aloft over the southwest. Onshore flow will strengthen Friday through the weekend, which will spread cooling inland and deepen the marine layer. Patchy night and morning coastal low clouds and fog will become more extensive and extend inland onto the coastal slopes of the mountains for the weekend. A low pressure system moving through California will spread rain across the area on Monday. Then dry and warmer for the middle of next week. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... At 1 PM, skies were mostly clear across Southern California. Temperatures are running about 2 to 8 degrees warmer than this time yesterday. Greater low cloud coverage is expected tonight, with low clouds reaching into coastal areas and locally into portions of the western valleys. A weak shortwave will pass to our north Friday and Saturday, strengthening onshore flow and spreading cooler temperatures inland. Marine layer clouds will become more widespread and extensive Friday night into Saturday morning, reaching inland onto the coastal slopes of the mountains. The marine layer may be deep enough for some patchy drizzle to fall Saturday morning. Little, if any, accumulation is expected. A deeper and more amplified trough is slated to impact California late this weekend and early next week, bringing widespread rainfall to the state. This trough will push south into central California early Monday and tap into atmospheric river moisture from the west. The atmospheric river will first impact central California, then weaken as it pushes southeast into SoCal on Monday. Higher rainfall totals are expected for central California, but we will still see widespread precipitation in SoCal on Monday. Broad southwest flow will enhance the upslope flow and the rainfall amounts in the San Bernardino mountains, where 1 to locally 1.5 inches of precipitation is forecast. In the Inland Empire and Orange County, rainfall amounts of 0.30 to 0.75 inches are forecast. Further south into San Diego County, much less rainfall is forecast, with only 0.10 to 0.30 inch expected, locally up to 0.50 inch in the San Diego mountains. Little, if any, measurable rainfall is expected in the lower deserts. Cluster analysis is coming into better alignment on the timing of this trough, but there is still a good amount of spread in the amplitude of the trough. The amplitude and position of this trough will be important in how much rainfall SoCal ends up receiving. The wettest ECMWF members are not quite as wet on the latest runs. Overall, forecast rainfall amounts have not changed much in the last day. Amounts are likely to change as we get closer to the event and models come into better alignment. This trough will tap into warm, subtropical atmospheric river moisture, so snow levels will remain quite high with this system, generally above 9,000 feet. Snow levels will drop to around 7,500 feet Tuesday morning once the system passes, but little, if any, precipitation will fall during that time. So this will be an all rain event for the mountain communities. A dusting of snow may occur on the highest peaks of the San Bernardino Mountains. Gusty southwest to west winds are expected with this system on Monday, with gusts to 40 mph possible through the passes and on the desert mountain slopes. The trough will exit to the east on Tuesday, with a weak ridge building in behind it. This will bring dry conditions and a warming trend for the middle of next week. Periods of weak offshore flow are possible towards the middle to end of next week. && .AVIATION... 212000Z...Except for mostly SCT clouds at/above 20000 feet MSL, clear skies with unrestricted vis will continue this afternoon. Some low clouds, with bases around 1000 feet MSL, will develop near the coast this evening after 03Z, which should reach SAN/CRQ/SNA after 05Z and spread locally 10-15 miles inland by sunrise Friday. Low clouds and fog are not expected to reach KONT or KSBD. Expect clearing to the coast 16Z-19Z Friday. && .MARINE... No hazardous marine conditions are expected through Sunday, although a few gusts around San Clemente Island could exceed 20 knots late Friday through Saturday. Stronger winds and higher seas are expected with a storm system arriving Monday. Stay informed with the Coastal Waters Forecast and any other hazard messages that may be issued. && .BEACHES... A series of west-northwest swells will boost surf and rip current risk today, Saturday, and again late Monday through Wednesday. Highest surf and rip current risk will be in southern San Diego County. Stay informed with the Surf Zone Forecast and any other hazard messages that may be issued. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...NONE. PZ...NONE. && $$ PUBLIC...Connolly AVIATION/MARINE/BEACHES...PG